Electrostatic printing wherein screen carries powder between loading and printing points



Sept. 12-, 1967 c. E. PILON 3,34

ELECTROSTATIC PRINTING WHEREIN SCREEN CARRIES POWDER BETWEEN LOADING ANDPRINTING POINTS Filed May 19, 1966 o o o m o o c iv POWDER CLOUD ICHAMBER POWDEPL\ v T RESERVOIR MoToR 4 POWDER CLOUD K GENERATOR v v I1\I ENTOR. CHARLES E. /L ON United States Patent 3,340,802 ELECTROSTATICPRINTING WHEREIN SCREEN CARRIES POWDER BETWEEN LOADHJG AND PRINTINGPOINTS Charles E. Pilon, Berkeley, Calif., assignor to ElectrostaticPrinting Corporation of America, San Francisco, Calif., a corporation ofCalifornia Filed May 19, 1966, Ser. No. 551,365 6 Claims. (Cl. 101114)ABSTRACT OF THE DISCLUSURE In an electrostatic printing system of thetype wherein electroscopic powder particles are urged through the openapertures of an image screen into an electric field to be carriedthereby through a substrate to form a powder image thereon, there isprovided an improved arrangement wherein the powder particles areapplied to the screen from a powder cloud chamber at a powder loadinglocation. Between the powder loading location and a powder transferringor a printing location, toward which the screen is moved, there isprovided a wiper or some other suitable means for removing the powderfrom the masked portions in the screen. At the printing, or powdertransferring location, a wiper is applied to the back surface of thescreen to mechanically actuate the screen whereby the powder particleson the opposite side of the screen are caused to leave the screen andmove into the electric field to be carried in the form of a powder imageto the receiving substrate. Provision is then made to make the wiperresistive or suitably conductive so that it functions to control theamount of powder which is removed from the screen into the electricfield.

-scopic powder particles employs a screen having all openings blockedoff except those which provide the desired image pattern. A conductiveplate is spaced from the screen and the substrate upon which it isdesired to print is inserted between the conductive plate and the screensurface. An electric field is established between the screen and theconductive plate. Electroscopic powder particles are urged through thescreen apertures into the field between the screen and the conductiveplate whereupon they are carried in the pattern of the image of thescreen to the substrate. The substrate is thereafter removed and theimage is fixed to the substrate by well known means, such as heat.

Various arrangements have been contemplated and used for moving thepowder particles through the openings of the screen into the electricfield. One of these arrangements establishes a highly charged powdercloud which is directed to the back side of the screen by an electricfield whereupon the powder particles move through the screen apertureswhich are open into the field between the substrate and the screen.Another means for applying powder is to use a brush to flick or rub thepowder particles through the screen openings. Still another arrangementuses a metal plate on which the powder particles are deposited and whichis brought to the back side of the screen. Upon the application of avoltage pulse between the plate and screen, the powder particles arecaused to move toward the screen under the influence of the electricfield which is established between the plate and the screen.

All of these arrangements have one common feature; the powder is broughtto the screen and transferred there through to the image receivingsubstrate at the same location and substantially simultaneously. Whilefor many applications this arrangement is satisfactory, for certainsituations, which occur in the electrostatic printing process, thistechnique of simultaneously bringing powder to the screen while printingcauses complications that preclude the designing of a simple and directprinting machine. For example, with rapidly moving conveyor lines oreven where it is necessary to move the product relative to the screen,it is difficult to bring powder to the screen and pass it through theapertures, either sufficiently rapidly or with sufficient quantityduring the interval within which printing is to occur. While suchfeeding equipment can be built, it is complicated and requires delicateadjustments and continual monitoring.

An object of this invention is the provision of a novel powder loadingand powder transferring system in an electrostatic printing system.

Still another object of this invention is the provision of a simplifiedsystem for loading powder on the screen of an electrostatic printingsystem at one location and transferring the powder to an image receivingobject at another location.

These and other objects of this invention are achieved in an arrangementwhereby powder is loaded onto an image screen over the apertures of theimage screen at a place remote from the actual place of printing. Thepreloaded image screen and the object to be printed on are then broughtinto the proper relationship with one another at a printing locationwhere the preloaded screen is effectively wiped to initiate the transferof the powder, in the pattern of the screen image, to the receivingobject.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe following description when read in connection with ,the accompanyingdrawings, in which:

FIGURE 1 is a schematic representation in elevation of an embodiment ofthe invention showing the preloading of powder at one side of thescreen;

FIGURE 2 is an enlarged representation in elevation of the screen andwiper blades of FIGURE 1 which is shown to illustrate how the screencarries the powder;

FIGURE 3 is a schematic representation in elevation of an embodiment ofthe invention showing another exemplary arrangement for preloadingpowder; and

' FIGURE 4 is an isometric view of an arrangement for preloading ascreen and printing therefrom in accordance with this invention.

Referring now to FIGURE 1, there may be seen a schematic representationin elevation of this invention. A conductive image screen 10,represented on edge, which has certain of its apertures blocked andothers of its apertures open, in accordance with a pattern desired to beprinted, is represented as moving in the directionof the arrows. Thescreen will be considered as having a printing side, the side which isopposite an object on which printing i to occur, and a back side, whichis the side opposite the printing side. The printing side of the imagescreen passes adjacent a powder cloud chamber 12, from whence chargedelectroscopic powder particles within a dense powder cloud establishedwithin said chamber, by well known fluidizing means, are urged towardthe screen 10 by virtue of an electric field which establishingpotential 14 which may be connected between either the conductive baseof the powder cloud chamber and the screen or between spaced wires (notshown) supported within the powder cloud chamber 12 and the screen. Theamount of powder which is deposited on the screen from the powder cloudchamber may be controlled by controlling the amplitude of the potential14 which establishes the electric field between the powder cloud chamberand the screen The powder particles adhere to the screen and fill thespace between the screen, in the manner shown in FIG- URE 2, and remainthere even though the screen is moving. As the screen moves out fromunder the powder cloud chamber it passes a wiper blade 16 which ispositioned on the printing side of the image screen for the purpose ofwiping off any of the excess powder which may extend too far beyond thesurface of the screen or which may be on the non-image portion of thescreen. It will be appreciated that powder which transfers to an objectfrom the non-image portion of the screen can spoil the image. The wiper16, by removing powder from all non-image screen areas, effectivelyinsures that the powder that remains is only in the image areas. Thus,all that remains to be done is to transfer the remaining powder, in theform of the image areas, to a receiving object or substrate.

The image screen then proceeds from the wiper blade 16 to the printingregion which is defined by the region between another preferablyresistive wiper blade 18 positioned at the back side of the screen andthe object 20 on which it is desired printing to occur. The object 20 isshown as circular, it may be a container upon whose surface it isdesired to print. Since it is circular, in order to print over thesurface it is necessary to rotate the container 20 at a speedsubstantially matching the speed of motion of the screen 10. An electricfield is established between the screen and the object 20 by the use ofa field establishing potential 22. A variable potential 24 is alsoconnected between the wiper blade 18 and the screen 10.

The wiper blade 18 mechanically contacts the screen and the wipingaction thereof on the screen together with the action of the electricfield established between the screen and the object 20 has the effect ofcausing the powder particles which are held at the screen and bridgingthe openings of the screen to be released from the screen to be moved bythe electric fiel-d onto the image receiving object 20. A furthercontrol on the amount of powder which moves through the apertures isestablished by the amplitude of the potential 24 which is applied to thewiper blade 18. This can hold at the blade 18 some of the powder whichmay be on the back side of the image screen. and otherwise would passthrough the apertures into the electric field between the screen and theobject 20. The screen passes out from the printing station and can becirculated, if so desired.

FIGURE 2 is an enlarged schematic drawing in elevation of a section ofthe screen 10 and the wiper blades 16 and 18, which are shown in orderto assist in an understanding of this invention. In order to constitutethe screen as the powder image forming electrodes cer tain areas of thescreen have their apertures blocked or masked by a suitable insulatingmaterial 11, which may be nylon or Teflon or any other suitableinsulator. Not only does this material fill the screen openings but italso extends a small distance away from the printing side of the screenwires. The reason for making the printing side of the screen the one inwhich the insulating material covers the screen wires and extends beyondis that it has been found that this eliminates multi-path activity whichoccurs with the more conductive powder particles which assume the chargeof the surface with which they are in contact and then are repelled bythe charge on the surface. The insulating material does not charge anypowder particles which happen to come in contact therewith, and thesepowder particles remain on the surface of the material.

The charged powder particles 13 which are directed by the electric fieldestablished between the powder cloud chamber and the screen, to thescreen, adhere to the screen wires, bridge their openings, and form oneor more layers in the manner shown in the drawing. The wiper 16 scrapesthe powder particles from the masking material of the screen, before thescreen reaches the printing area, leaving the powder only covering theimage areas of the screen. A container, not shown, may be employed atthe base of the wiper 16 to retrieve the excess powder removed by thiswiper blade.

At the printing area, the mechanical wipe of the screen wire by thewiper 18 dislodges the powder on the screen wires. Some of the powder isalso dislodged by the action of the electric field established betweenthe screen wires and the backplate or object being printed on. All ofthe dislodged powder is directed by the electric field which isestablished between the screen wires and the backplate or the objectbeing printed on, to the Object. The potential 24 which is applied tothe Wiper blade 18, may be applied across the wiper blade as shown inFIG- URE 1, or may be applied to a conductive wire 19, embedded in theresistive material of the wiper blade.

FIGURE 3 shows still another arrangement for preloading a screen 30.This time a brush 32 rotates so that its fibers can receive a load ofpowder from a powder reservoir 34. The brush continues to rotate untilits fiber or bristles deposit the powder in the openings of an imagescreen 30. In order to control the amount of powder which is depositedand to retain the powder at the screen, a source of potential 36 is usedto apply a bias voltage between the hub of the brush and the screen 30.As the screen continues to move away from the loading region, the imageprinting side of the screen is wiped clean by a wiper blade 38. Thescreen 30 moves into a printing position where this time the wiper is aroller 40 instead of a fixed wiper blade. The operation of the inventionis the same as was previously described. The roller causes a mechanicalagitation of the screen sufficient to loosen the powder particles at thescreen openings 50 that under the influence of the electric fieldestablished between the object 42 and the screen by the field potentialsource 44 connected therebetween, the powder particles are depositedupon the object. A bias potential 46 may be connected between the rollerand the screen, if the roller is made resistive for the purpose ofdetermining how much of the powder which is loosened with the screenopenings, is transferred to the receiving object 42.

FIGURE 4 shows an isometric view illustrative of an arrangement of theinvention. The screen 50 is in webbed form and is stretched betweenrollers 54, 56. The motor 58 rotatably drives the roller 56 whereby thescreen is continuously cycled by the rotation of the roller in thedirection indicated by the arrows on the roller. A powder cloudgenerator 60 loads the openings of the screen with powder particles. Thebias for transferring powder between the powder cloud chamber and thescreen is provided by the bias source 62 applied between the generator60 and a conductive plate 61 on the other side of the screen 50. A wiperblade 64 wipes the printing side of the screen 50 clean and any powderwhich is wiped off the screen is collected in a container 66.

The screen next moves to the printing position where a wiper blade 68presses down on the screen to cause the powder particles which are inthe image areas thereof to leave the screen and to be directed towardthe object 72 which is being printed on. A source of bias potential 70is connected between the wiper 68 and the screen 50. The electric fieldrequired for transferring the powder between the screen and the object72 is provided by the bias source 74-, connected between the roller andthe screen. After the screen moves through the printing station, it isturned around the roller 56 and the roller 54 to assume another powderload from the powder cloud chamber.

While the foregoing description of the embodiment of the invention isone wherein the screen moves past the powder loading station and thenpasses the powder image transfer station it should be realized that itis well within the scope of this invention as well as within the skillof those versed in the art to use a stationary screen with the powderloading arrangement being made movable together with the wiper at theprinting position while the object being printed on is made stationary.Also, the screen need not be a continuous one which extends between thepowder loading position and the powder image transfer position, butrather can be individual screen sections which are loaded and thereafterbrought to the powder image transfer region for unloading.

While the wiper blade or roller at the printing position is shown with apotential applied between it and the screen for the purpose of assistingin controlling the amount of powder transferred at the image transferlocation, it should be appreciated that it is also within the scope ofthis invention to omit the use of such potential and rely solely on themechanical dislodgement of the powder by the wiper. The pressure appliedby the wiper or roller on the screen at the image transfer locationshould not be such as will deform the screen but rather should be enoughso that the relative motion between the wiper and the screen causes somemovement of the individual strands of the screen such that the powderparticles are dislodged solely mechanically or as a result of thecombined mechanical and electrical forces.

There has accordingly been described and shown herein a novel, usefuland simple arrangement for preloading the screen, and thereafter fortransferring powder from the screen at a printing location.

What is claimed is:

1. In an electrostatic printing system, a conductive apertured screenhaving a printing side and a back side which is opposite said printingsurface, a masking material blocking a portion of the screen aperturesto leave the remaining unblocked apertures arranged in the form of adesired image, said masking material extending beyond said printing sideof said screen, means establishing a screen loading location, meansestablishing a screen unloading location displaced from said screenloading location, means for relatively moving said screen between saidscreen loading location and said screen unloading location, said meansestablishing a screen loading location including a source ofelectroscopic owder particles, each of said powder particles beingsmaller than the apertures of said screen, means for transferring powderfrom said source onto said printing side of said screen, cleaning wiperblade means positioned between said screen loading and unloadingposition for removing powder particles deposited over said screenmasking material at said screen loading location, said meansestablishing said screen un loading location including means forestablishing an electric field between said screen and an imagereceiving object positioned adjacent said printing side of said screen,and printing wiper means in contact with the back side of said screen,said printing wiper means mechanically agitating and Wiping the backside of said screen for mechanically dislodging the powder particles onsaid screen into the electric field between said screen and an imagereceiving object to be moved toward said image receiving object underthe influence of said electric field.

2. In an electrostatic printing system a recited in claim 1 wherein saidprinting wiper means comprises a wiper blade made of resistive materialand there is included a source of potential and means connecting saidsource of potential between said screen and said wiper blade.

3. In an electrostatic printing system as recited in claim 1 whereinsaid printing wiper means comprises a roller in frictional engagementwith said screen, and there is provided a source of electric potentialand means for connecting said source of electric potential between saidroller and said screen.

4. In an electrostatic printing system as recited in claim 1 whereinsaid printing wiper means comprises a wiper blade having a conductoradjacent the tip thereof, said conductor being coextensive with saidscreen, said means for establishing an electric field between saidscreen and said object includes a source of electric potential, andmeans for connecting said conductor and said object to said source ofelectric potential.

5. Apparatus as recited in claim 1 wherein said source of electroscopicpowder particles comprises means for generating a cloud of electroscopicpowder particles positioned at one side of said screen, and said meansfor transferring powder particles from said source onto said screenincludes a source of electric potential, and means for connecting saidsource of electric potential between said powder cloud forming means andsaid screen for establishing an electric field there between to transferpowder particles from said means for establishing a cloud of powderparticles to said screen.

6. In an electrostatic printing system as recited in claim 1 whereinsaid means for transferring powder particles from said source onto saidscreen includes a rotatable brush having bristles extending from aconductive hub and being positioned for bringing its bristles intocontact with the surface of said screen, a source of electric potenital,and means for connecting said source of electric potential between thehub of said brush and said screen for controlling the amount of powderto be transferred between said brush and said screen.

References Cited UNITED STATES PATENTS 2,824,813 2/1958 Fauser et al.2,959,153 11/1960 Hider. 2,966,429 12/ 1960 Darrel et al. 3,081,6983/1963 Childress et al. 3,152,012 10/ 1964 Schaffert. 3,220,833 11/ 1965McFarlone. 3,245,341 4/1966 Childress et al. 3,253,540 5/1966 Lusher.3,261,284 7/1966 Lynott et al.

ROBERT E. PULFREY, Primary Examiner,

E. S. BURR, Assistant Examiner.

1. IN AN ELECTROSTATIC PRINTING SYSTEM, A CONDUCTIVE APERTURED SCREENHAVING A PRINTING SIDE AND A BACK SIDE WHICH IS OPPOSITE SAID PRINTINGSURFACE, A MASKING MATERIAL BLOCKING A PORTION OF THE SCREEN APERTURESTO LEAVE THE REMAINING UNBLOCKED APERTURE ARRANGED IN THE FORM OF ADESIRED IMAGE, SAID MASKING MATERIAL EXTENDING BEYOND SAID PRINTING SIDEOF SAID SCREEN, MEANS ESTABLISHING A SCREEN LOADING LOCATION, MEANSESTABLISHING A SCREEN UNTION, MEANS FOR RELATIVELY MOVING SAID SCREENBETWEEN SAID SCREEN LOADING LOCATION AND SAID SCREEN UNLOADING LOCATION,SAID MEANS ESTABLISHING A SCREEN LOADING LOCATION INCLUDING A SOURCE OFELECTROSCOPIC POWDER PARTICLES, EACH OF SAID POWDER PARTICLES BEINGSMALLER THAN THE APERTURES OF SAID SCREEN, MEANS FOR TRANSFERRING POWDERFROM SAID SOURCE ONTO SAID PRINTING SIDE OF SAID SCREEN, CLEANING WIPERBLADE MEANS POSITIONED BETWEEN SAID SCREEN LOADING AND UNLOADINGPOSITION FOR REMOVING POWDER PARTICLES DEPOSITED OVER SAID SCREENMASKING MATERIAL AT SAID SCREEN LOADING LOCATION, SAID MEANSESTABLISHING SAID SCREEN UNLOADING LOCATION INCLUDING MEANS FORESTABLISHING AN ELECTRIC FIELD BETWEEN SAID SCREEN AND AN IMAGERECEIVING OBJECT POSITIONED ADJACENT SAID PRINTING SIDE OF SAID SCREEN,AND PRINTING WIPER MEANS IN CONTACT WITH THE BACK SIDE OF SAID SCREEN,SAID PRINTING WIPER MEANS MECHANICALLY AGITATING AND WIPING THE BACKSIDE OF SAID SCREEN FOR MECHANICALLY DISLODGING THE POWDER PARTICLES ONSAID SCREEN INTO THE ELECTRIC FIELD BETWEEN SAID SCREEN AND AN IMAGERECEIVING OBJECT TO BE MOVED TOWARD SAID IMAGE RECEIVING OBJECT UNDERTHE INFLUENCE OF SAID ELECTRIC FIELD.